This invention relates generally to compositions for finishing and preserving lithographic printing plates, more particularly to single-phase aqueous composition formulations that include as their hydrophilic component a blend of natural gum and synthetic resins in specific combination and admixture with a preservative, a buffer system, and a surfactant blend, each of which enhance the advantageous properties of the overall system while being compatible with each other and with the resin blend.
When a negative-acting lithographic plate is exposed to actinic radiation, the diazonium and/or polymer compositions coated thereon are hardened or insolubilized. When a positive-acting lithographic printing plate is exposed to actinic radiation, the diazonium and/or polymer coating thereof becomes softened or increased in solubility. After either of these types of plates is thus imaged by exposure to actinic radiation, it is developed by applying thereto a developer which is a solvent for the unexposed polymer and/or diazonium material and thereafter removing the developer together with the solubilized polymer and/or diazonium material. The thus developed plates will accept ink where the polymer and/or diazonium coating remains and will reject ink where the coating has been dissolved and removed by the developer. Typically, a finisher is then applied to the developed lithographic printing plate in order to protect the non-image areas and to condition the image area of the plate to maintain ink receptivity while the plate is stored prior to actual printing therewith.
A serious drawback of most lithographic plate finishers is the appearance of a blinding condition over the image area caused by including within finisher formulations a resin such as gum arabic, which is the most widely used finishing agent. Highly viscous resins such as gum arabic tend to lay an uneven, streaky film, especially when applied by a mechanical plate processor, that thereby can "blind" the image area. Known synthetic gum finisher formulations can also cause blinding problems unless they are applied with extreme care and dexterity at a level of skill that is usually not attainable by a mechanical plate processor. Additionally, many plate finishers are specially formulated for use on lithographic plates having a particular coating chemistry, which significantly limits the usefulness of such finishers with respect to exposed plates other than those of such special formulation.
A disadvantage of emulsion finishers, such as those of U.S. Pat. No. 4,162,920, is that they tend to separate on storage and when applied to imaged lithographic plates thereby hampering their usefulness in finishing plates while avoiding blinding, scumming and extended roll-up cycles. Emulsion finishers also have a cost disadvantage when compared with aqueous finishers that include larger relative quantities of water within the finisher compositions.
Accordingly, there is a need for a single-phase aqueous finisher composition having high quantities of water that will finish a wide variety of image lithographic plates by desensitizing and preserving the hydrophilic non-image areas and protecting the oleophilic image areas while substantially eliminating blinding and scumming and while accomplishing acceptable roll-up through a minimal number of cycles. These and other advantageous properties are provided by the finisher and preserver according to this invention which is a single-phase aqueous composition including a blend of natural and synthetic hydrophilic resins, a preservative for such natural resin, a buffer system of phosphate and phosphoric acid, a surfactant composition that is preferably a particular blend of surfactants, and at least about 85% water, based on the total weight of the aqueous composition.
It is, therefore, a general object of the present invention to provide an improved aqueous finisher composition for lithographic plates.
Another object of the present invention is to provide a single-phase, aqueous finisher and preserver for a wide variety of lithographic plates having any of various coating compositions responsive to actinic radiation.
Another object of this invention is to provide an aqueous finisher composition having a low viscosity and exhibiting good flow properties.
Another object of the present invention is to provide an improved plate finisher and preserver that affords exceptionally rapid roll-up of imaged lithographic plates treated therewith.
Another object of this invention is an improved finisher and preserver composition that exhibits a broad temperature and pH stability over the range of temperature and pH values typically encountered during commercial printing operations.
Another object of this invention is an improved finisher and preserver that has an indefinite shelf life, that buffs up dry and clear when used on an imaged lithographic plate, and that does not dry up within a lithographic plate processing machine.
Another object of the present invention is an improved aqueous composition that finishes and preserves imaged lithographic plates to the extent that they can be stored for greater than eight weeks without evidence of blinding or scumming.
Another object of this invention is an improved finisher and preserver for lithographic plates that does not clog holes within the spray tubes of mechanical plate processors and that does not harden and cake within the processor.
Another object of this invention is to provide an improved single-phase aqueous finisher and preserver composition for lithographic plates that is compatible with a wide variety of press fountain solutions.
These and other objects of this invention will be apparent from the following further detailed description thereof.
The finisher and preserver composition in accordance with this invention is a single-phase aqueous composition including between about 2 and 12 total weight percent, preferably between about 3 and 8 weight percent, of a blend of natural gum and synthetic hydrophilic resins, between about 0.05 and about 1 weight percent of a preservative for such natural resin, between about 0.5 and about 5 weight percent of a phosphate and phosphoric acid buffer system, between about 0.01 and about 0.2 weight percent of a surfactant composition, and between about 85 and 95 weight percent water, all percentages being based upon the total weight of the single-phase aqueous composition.
Such blend of natural and synthetic resins includes a natural gum resin such as gum arabic which, if used alone, would readily lead to blinding problems. It has been discovered that by blending a natural gum resin such as gum arabic with certain synthetic resins, a resin blend is provided which is hydrophilic for protecting non-image areas from oxidation and scumming while simultaneously preventing image-area blinding and poor roll-up that is often associated with compositions containing a single resin. A particularly advantageous blend is that of between about 1 and 5 weight percent, preferably between about 2 and 4 weight percent, of a natural gum resin, together with between about 0.5 and 5 weight percent, preferably between about 0.8 and 3 weight percent, and most preferably between about 1 and 2 weight percent of a cyclic amide synthetic resin between about 0.1 and 2 weight percent, preferably between about 0.3 and 1 weight percent of a modified polyacrylamide synthetic resin. While the synthetic resins alone do not adequately protect the non-image areas of the exposed plate, they do improve the water solubility of the resin blend, enhance the breadth of the temperature and pH stability of the overall composition, and combine with other compounds of the composition to increase the stability of the single-phase composition.
Because of its ready availability and wide acceptance, gum arabic is the preferred natural resin, and the preferred synthetic resins are of the cyclic amide type and of the hydrolyzed polyacrylamide type. Especially preferred are polyvinyl pyrollidone and a polyacrylamide synthetic resin of a molecular weight on the order of about 200,000 that is between about 60 and 70 percent hydrolyzed to acrylic acid groups. Especially preferred is the combination of these two types of synthetic resins with the natural resin, the preferred formulation being one having 6 parts by weight of gum arabic, 3 parts by weight of polyvinyl pyrollidone, and 1 part by weight of an approximately 70% hydrolyzed polyacrylamide.
Preservatives are preferably included within the composition in order to protect the composition from growth of molds which tends to occur in formulations incorporating natural gums or resins. Such preservatives are included in a mold growth-inhibiting amount; for example, a preservative such as the preferred sodium benzoate, which is especially desirable because it does not pose any pollution problems, will typically be present within the formulation within a range of between about 0.05 to about 1 weight percent, preferably between about 0.1 and about 0.5 weight percent, based upon the weight of the total composition.
In order to maximize the stability of the total composition, a buffering system is provided for reducing the likelihood of undesirable precipitation by maintaining the pH of the composition to within close tolerances. The buffering system in accordance with this invention also assists in desensitizing the non-image areas in order to prevent undesirable ink adherence and scumming, while preferably also being effective as a plate cleaner. All such properties are imparted to the finisher and preserver composition of this invention when it incorporates a buffer system combining a phosphate and phosphoric acid, the preferred buffering system being a combination of monosodium phosphate, trisodium phosphate, and 85% phosphoric acid.
The concentration of the buffering system will vary somewhat depending upon what pH is needed to enhance the stability of any particular specific formulation of the finisher and preserver composition according to this invention; for example, the preferred pH for a particular advantageous composition formulation is a pH of 4.3, and the preferred buffering system maintains the pH between about 4.1 and 4.5 under normal storage and use conditions. In addition to accomplishing this needed pH control, the buffering system according to this invention also assists, particularly by the inclusion of trisodium phosphate at its preferred concentration ratio, in the plate cleaning aspects of this invention; the buffering system further aids, particularly by the inclusion of phosphoric acid at its preferred concentration ratio, in desensitizing the non-image areas of the exposed plate and maintaining them hydrophilic as well as in preventing ink adherence and scumming. Particularly preferred is a buffering system including monosodium phosphate, trisodium phosphate and phosphoric acid at a weight ratio of 1:3:3 within an overall concentration range of between about 0.5 and 5 weight percent, preferably between about 1 and 3 weight percent of the total aqueous composition, the concentration of monosodium phosphate generally ranging between about 0.1 and 0.5 weight percent, and the concentration of each of trisodium phosphate and phosphoric acid generally ranging between about 0.3 and 2 weight percent thereof.
Surfactants are typically included within the composition, preferably those having low foaming properties and good wetting capabilities, especially with respect to the image-area polymeric materials, in order to enhance the uniformity of the finisher and preserver coating of this invention onto the imaged lithographic plate to assist in protecting the non-image areas of the exposed plate. Suitable low foaming or non-foaming surfactants include modified linear alcohol ethoxylates and modified aliphatic polyethers, preferably in combination with each other at a weight ratio on the order of 1:1 and which are typically present within the composition of this invention at a total concentration of between about 0.01 and 0.2 weight percent, preferably between about 0.02 and 0.1 weight percent.
In proceeding with the method according to this invention, a natural gum resin and one or more synthetic resins are blended into water until the natural and synthetic resins are substantially totally dissolved therewithin. Typically, the total natural and synthetic blend of resins will be dissolved within water at a weight ratio of between about 1:50 and about 1:6. The remaining compounds of the composition, including the phosphate and phosphoric acid buffering system, the surfactant constituent, and the preserver compound, are usually more water soluble than the natural and synthetic resins, and these may be added directly to the water within which the natural and synthetic resin blend is dissolved or they may be incorporated into a smaller volume of water for subsequent combination with the dissolved resin blend aqueous system. By this method, a single-phase aqueous solution is prepared which includes between about 2 and 12 weight percent of a blend of natural and synthetic resins, between about 0.05 and about 1 weight percent of a preserver compound for the natural resin, between about 0.5 and about 5 weight percent of a phosphate and phosphoric acid buffering system blend, between about 0.01 and about 0.2 weight percent of a low-foaming surfactant constituent and between about 85 to about 95 weight percent water, all percentages being based upon the weight of the total aqueous composition.
In use, the composition is applied to an exposed and developed lithographic plate. When such application is made by hand, preferably a small amount thereof is poured onto the plate and is spread with a webril wipe or a damp sponge, after which the plate is preferably thoroughly buffed dry with a clean, absorbent wiping cloth or pad. When the finisher and preserver composition is used within a mechanical plate processor, the composition is added to the proper level within the finisher reservoir of the machine. During operation of the machine, the compositions do not foam undesirably upon agitation, and they spread uniformly over the imaged and developed lithographic plate by the brushes or sponges of the particular mechanical plate processor being used. The compositions will not dry up in these processors, and since they possess a relatively low viscosity, on the order of 20-25 centipoises, they will not clog spray tubes within the processor, even when the processor has been shut down for extended time periods while the composition of this invention is left therewithin.
Compositions prepared according to this invention have an indefinite shelf life, and will remain in storage within the bottle for two to three years without evidence of mold growth, without substantial separation of the single phase, and without evidence of component hydrolyzation. When used on the plates in accordance with the method of this invention, the imaged, developed and finished plates are preserved to prevent deterioration or damage of the image areas and to maintain the desensitization of the non-image areas, such preservation being effective for on the order of three months without evidence of blinding or scumming upon printing with such plates.